How LDO and HSD fired package boilers work
Light diesel oil and high speed diesel fire small package boilers, typically 0.5 to 10 TPH, often as standby to biomass or gas boilers, or where heating heavy oil in storage is not justified. Both are distillate fuels that flow at ambient temperature and need no heating. They are atomised by pressure jet nozzles in a monoblock burner and have a calorific value near 10,500 kcal/kg. Modern HSD is very low in sulphur; LDO can carry more. Combustion is clean, with little soot while the nozzle is in good order. The boiler fires under positive pressure and the burner fan alone drives the gas to the chimney; an ID fan is not normal practice.
Where the FD fan sits
- Boiler house air
- FD fan
- Burner air damper
- Blast tube
- Furnace
- Tube passes
- Chimney
The fan is part of the monoblock burner, with one motor driving the fan and often the oil pump on a common shaft. It draws air from the boiler house through an inlet damper and pushes it down the blast tube to the flame. Fan pressure covers the burner head, the furnace and all tube passes, typically 50 to 150 mmWC on a small three-pass boiler. Larger units of 5 to 10 TPH may use a separate fan with a dual-block burner.
Air at the FD fan
| Air temperature at the fan | Boiler house air, 25 to 45°C |
| Pressure at the fan outlet | Typically 50 to 150 mmWC |
| Firing stages | Two or three stages on nozzles, or modulating on larger burners |
| Dust | Clean; boiler house dust where biomass is handled nearby |
| Purge flow | Full air for a timed pre-purge before every start |
| Density at the design temperature | About 1.13 kg/m³ at 40°C, 94% of standard air. Pressure developed and power drawn scale with density, so the fan is selected at this condition and the motor is checked at the coldest operating or start-up condition. |
FD Fan selection for LDO and HSD fired package boilers
| Blade form | Forward curved on small monoblock burners, backward curved on larger separate fans |
| Material of construction | Aluminium or mild steel |
| Drive and arrangement | Direct mounted on the burner motor, often shared with the oil pump |
| Bearings and cooling | Motor bearings |
| Flow control | Air damper with servo or hydraulic stage actuator; VFD on larger burners |
| Also specified | Silencer hood, inlet screen, pressure switch proving air flow |
What this FD fan has to cope with
Back pressure above the burner's firing curve
Monoblock burners are chosen from a firing diagram of output against furnace pressure. If the boiler has a long flue, an economiser or an undersized chimney, back pressure exceeds the curve and the burner cannot reach full output. A separate fan with more pressure, or a lower flue resistance, restores the rating without overfiring the nozzles.
Frequent cycling and purge losses
As standby boilers, LDO and HSD units often run at low load with many starts per hour. Each start begins with a full-air purge that sweeps heat out of the boiler. A modulating burner with a VFD fan, or a smaller burner matched to the real load, cuts the number of starts and the heat lost with them.
Air proving switch trips
A dirty inlet screen, a loose impeller on its shaft or a misadjusted damper drops fan pressure below the air proving setting, and the burner locks out. Keeping the inlet clean, checking the impeller fixing at service and setting the proving switch with margin below the actual low-fire pressure prevent nuisance trips.
Energy and control
Many LDO and HSD boilers are standby units or run only at peak demand, so annual fan energy is small but fuel cost is high. The priority is excess air: a staged burner with a fixed damper often runs at 5 to 6% O₂. A modulating burner with a VFD fan and O₂ trim brings this nearer 3% and pays back through fuel saving rather than through fan power.
Questions buyers ask
What type of fan is used in an HSD fired boiler burner?
Small monoblock burners usually have a forward curved or compact backward curved fan mounted on the burner motor. Larger boilers use a separate backward curved fan feeding a dual-block burner. Both supply clean air and push the gas through the boiler under pressure, so their selection is about pressure and air control, not temperature.
Why can my LDO fired boiler not reach full steam output?
The common causes are worn or wrong nozzles, low oil pressure, or a burner fan that cannot overcome the boiler back pressure, often after an economiser or a longer flue was added. Checking furnace pressure at full fire against the burner firing diagram shows whether the fan is the limit.
Do LDO and HSD boilers need an ID fan?
No, not in normal practice. The burner fan pressurises the furnace and drives the gas to the chimney. An ID fan is only considered where the flue route is unusually long or there is equipment after the boiler, such as a heat recovery unit, with significant resistance that the burner fan cannot overcome.
Information needed to quote an FD fan for LDO and HSD fired package boilers
- Duty point. Combustion air quantity in m³/h or kg/h and the static pressure at the fan outlet, with the margins you want.
- Air condition. Site ambient temperature range and altitude, and whether the fan takes ambient air or air already heated.
- System. Boiler or furnace capacity and fuel, resistance of the air heater, windbox, grate or burner, and the existing fan curve or nameplate if this is a replacement.
- Mechanical. Drive arrangement, flow control (damper, inlet vanes or variable speed drive), noise limit, rotation and discharge orientation.
- Burner and firing stages. Burner rating, and whether it fires in stages or modulates.
- Flue route. Flue length, bends, economiser and chimney height after the boiler.
Built and tested in Chennai
Adroit Fantech Engineers has designed and manufactured industrial fans at Ayanambakkam, Chennai since 2004. Each impeller, casing and shaft is built for the gas and temperature it will see, performance tested as per IS 4894 or BS 848 Part I, and balanced to grade G 6.3 or G 2.5 as per ISO 21940-11, with the records supplied with the fan. We also replace rotors and retrofit existing fans of any make.